scope:

1. Introduction

Collapse strength of casing is one of the most important performance properties of oil country tubular goods, particularly for deep wells. There is no tension at the bottom of casing string, but the axial tension acting on the casing grows due to the weight of casing string below it in proceeding upward. So, the collapse strength of casing, especially for deep wells, must be studied under combined external pressure and axial load.

A Work Group, under chairman Kerr Godfrey, studied on the effect of axial loading on collapse resistance of casing, and has obtained the conclusions that the formula (1.1.5.1) in API Bul 5C3 gives an ultra conservative estimation for collapse resistance of pipe subjected to axial loading, but that past data are not enough for developing a new collapse formula. Then, a test program has been planned to elucidate the effect of axial loading on collapse for commercial casings.

The present report shows the results of collapse test according to the test program, which was contracted between American Petroleum Institute and Nippon Steel Corporation, as a part of API PRAC Project 81-38: "Collapse Testing of Casing with Combined Axial Loading".